A dermatologist shares 3 ways to decide if your old skincare products are safe to use, or if you should throw them out

• Most beauty products have expiration dates or PAO (Period After Opening) symbols.
• Skincare items can get expensive, and it's tempting to use them past the date.

We've all been there: You splurge on a tiny bottle of high-quality hyaluronic acid. Because of the steep price, you want to use it sparingly, which just leads to forgetting about it for months.

Cut to: It's been over a year (or maybe closer to two) since you opened it, with more than half a vial of liquid left. Do you toss it out, calculating about $60 lost, or do you risk it and use it anyway?

"The biggest risks of using expired skincare products include loss of efficacy and irritation from degrading ingredients," Dr. Ivy Lee, a board-certified dermatologist practicing in Los Angeles, told Insider.

Lee said liquid or semi-liquid products (including makeup) can pose a higher risk of causing irritation because water-based products can act as breeding grounds for bacteria.

According to the FDA, cosmetic products usually contain preservatives that control the growth of bacteria or fungi. Over time, those preservatives break down and lead to bacterial or fungal overgrowth, especially if you store your products in a moist, warm bathroom.

But even without any breakouts or rashes, you just won't be getting the true benefits of your vitamin C serum or retinol if you're using it after the expiration date.

For new products, use a marker to write down when you opened it

As with food, most beauty and skincare products come with expiration dates or PAO (Period After Opening) symbols that indicate how long you should use the product after that first pump.

Since it can be hard to keep track of all your products, Lee suggested keeping a permanent marker nearby and simply writing down the date you opened the product, ideally next to the PAO symbol.

You can also do some quick math and scrawl down when to throw out the item if that's easier.

Look for funny smells or big changes in consistency

If you forgot how long you've been using a product, you can also look for signs it expired. Like you would with food, Lee recommended being on the lookout for any unusual smells or changes in how the formula feels, especially if it's liquid or semi-liquid, which is more likely to lose its consistency.

For example, if your skin cream has separated (i.e., water drips out when you squeeze the tube), you're better off tossing it out.

Store your new products wisely

The FDA advises against buying resold skincare products over the internet — you might not know how fresh that cleanser actually is.

It also warns against leaving products like sunscreen out in the sun, which can degrade them faster from the heat.

All in all, most skincare products should last you at least six months after opening, depending on the formula and container. That's why it's great to be mindful of what you buy and how often you actually think you'll use it, to prevent the heartbreak of throwing out that half-used lotion you loved.